Atelosteogenesis Type 2

[McAlister Dysplasia, de la Chapelle Dysplasia]

Clinical Diagnosis

Atelosteogenesis type II (AO2) is usually lethal at birth or shortly thereafter because of pulmonary hypoplasia and tracheobronchomalacia. The diagnosis is suspected when the following are present:

Clinical features

• Rhizomelic limb shortening with normal-sized skull

• Hitchhiker thumbs

• Small chest

• Protuberant abdomen

• Cleft palate

• Distinctive facial features (midfacial hypoplasia, depressed nasal bridge, epicanthus, micrognathia)

Other usual findings are ulnar deviation of the fingers, gap between the first and second toes, and clubfoot.

Radiographic findings

• Normal size skull with disproportionately short skeleton

• Platyspondyly, hypodisplastic vertebrae, and cervical kyphosis. Ossification of the upper thoracic vertebrae and coronal clefts of the lumbar and lower thoracic vertebrae may be incomplete.

• Hypoplastic ilia with flat acetabulum. The pubic bones are often unossified.

• Shortened long bones with metaphyseal flaring. The distal humerus is sometimes bifid or V-shaped, sometimes pointed and hypoplastic; the femur is distally rounded; the radius and tibia are typically bowed. A distally pointed, triangular humerus had led Slaney et al (1999) to the suggestion of a new condition, but this finding is a typical feature of achondrogenesis 1B (ACG1B) bordering on AO2 [Unger et al 2001]. The first individuals with de la Chapelle dysplasia described by De la Chapelle et al (1972) and Whitley et al (1986) showed a triangular remnant of ulna and fibula. Those individuals were subsequently classified as having AO2.

• Characteristic hand findings of sulfate transporter-related dysplasia:

  • Hitchhiker thumb with ulnar deviation of the fingers [characteristic of diastrophic dysplasia (DTD)]

  • Gap between the first and second toe [characteristic of ACG1B (when the phalanges are identifiable on the x-rays) and DTD]

  • Hypoplasia of the first metacarpal bone (also present in ACG1B and DTD)

Testing

Histopathologic testing. The histopathology of cartilage is essentially similar to that seen in diastrophic dysplasia (DTD) and achondrogenesis 1B (ACG1B), as it reflects the paucity of sulfated proteoglycans in cartilage matrix [Superti-Furga, Hastbacka, Rossi et al 1996; Rossi et al 1997]. It shows an abnormal extracellular matrix with threads of fibrillar material between cystic acellular areas and areas of normal cellularity. Some chondrocytes appear surrounded by lamellar material forming concentric rings that are in some cases indistinguishable from the collagen rings typical of ACG1B. The growth plate shows disruption of column formation and hypertrophic zones with irregular invasion of the metaphyseal capillaries and fibrosis. These cartilage matrix abnormalities are present in long bones as well as in tracheal, laryngeal, and peribronchial cartilage, whereas intramembranous ossification shows no abnormalities.

Biochemical testing. The incorporation of sulfate in macromolecules can be studied in cultured chondrocytes and/or skin fibroblasts through double labeling with ³H-glycine and ³⁵S-sodium sulfate. After incubation with these compounds and purification, the electrophoretic analysis of medium proteoglycans reveals a lack of sulfate incorporation [Superti-Furga 1994, Rossi et al 1997], which can be observed even in total macromolecules. The determination of sulfate uptake is possible but very cumbersome and is not used for diagnostic purposes [Superti-Furga, Hastbacka, Wilcox et al 1996].

Molecular Genetic Testing

GeneReviews designates a molecular genetic test as clinically available only if the test is listed in the GeneTests Laboratory Directory by either a US CLIA-licensed laboratory or a non-US clinical laboratory. GeneTests does not verify laboratory-submitted information or warrant any aspect of a laboratory's licensure or performance. Clinicians must communicate directly with the laboratories to verify information.—ED.

Gene. SLC26A2 (DTDST) is the only gene currently known to be associated with atelosteogenesis type II (AO2).

Clinical uses

• Confirmatory diagnostic testing

• Carrier testing

• Prenatal diagnosis

Clinical testing. In individuals with radiologic and histologic features compatible with the diagnosis of AO2, mutations in the SLC26A2 gene can be found in more than 90% of alleles [Rossi & Superti-Furga 2001]. Occasionally, in individuals with typical clinical, radiologic, and histologic features of SLC26A2-related dysplasia (even with evidence of defective sulfation in fibroblasts), SLC26A2 molecular testing detects no mutation or only a single heterozygous pathogenic mutation; in these cases, the mutations may be present in the 5' region of the gene, which is not entirely covered by current clinically available testing.

• Targeted mutation analysis. Testing for the five most common SLC26A2 mutations (p.R279W, IVS1+2T>C ("Finnish"), p.C653S, delV340, and p.R178X) is available on a clinical basis. Of those, the p.R279W, IVS1+2T>C, and p.R178X are associated with the AO2 phenotype.

• Sequence analysis. Sequence analysis of the SLC26A2 coding region is available on a clinical basis. Sequence analysis may detect rare "private" mutations in individuals in whom mutation analysis detects none or only one of the common alleles.

Table 1 summarizes molecular genetic testing for this disorder.

Table 1. Molecular Genetic Testing Used in Atelosteogenesis Type II

Test Method	Mutations Detected	Mutation Detection Rate	Test Availability
Targeted mutation analysis	Panel of five SLC26A2 mutations 1	~65%	Clinical
Sequence analysis	Private and common SLC26A2 mutations	>90%

1. p.R279W, IVS1+2T>C, delV340, p.R178X, p.C653S

Interpretation of test results. For issues to consider in interpretation of sequence analysis results, click here.

Testing Strategy for a Proband

Clinical and radiologic features can strongly suggest the diagnosis of AO2, but histolopathology of cartilage, molecular genetic testing, and biochemical testing also provide important information.

Histolopathology of cartilage is particularly important when radiologic material is not available or is of poor quality.

Molecular genetic testing is the preferred confirmatory diagnostic test in probands with a clinical, radiologic, and/or histopathologic diagnosis of AO2; it allows a precise diagnosis in the great majority of cases.

• Targeted mutation analysis for the five most common mutations is performed first, as it is likely to identify one or both alleles in a significant proportion of probands (one allele in 33% and both alleles in about 25%).

• Sequence analysis of the entire coding region is performed when neither or only one allele has been identified by targeted mutation analysis. Parental DNA analysis for the mutations found in the proband is recommended, as most probands are compound heterozygous.

Sulfate incorporation assay in cultured skin fibroblasts (or chondrocytes) is possible in the rare cases in which the diagnosis of AO2 is strongly suspected but mutation analysis fails to detect SLC26A2 mutations.

Genetically Related (Allelic) Disorders

Three other phenotypes (all with an autosomal recessive mode of inheritance) are associated with mutations in SLC26A2:

• Achondrogenesis 1B (ACG1B), among the most severe skeletal disorders in humans, is characterized by severe hypodysplasia of the spine, rib cage, and extremities, with a relatively preserved cranium. ACG1B is invariably lethal in the perinatal period [Superti-Furga, Hastbacka, Wilcox et al 1996].

• Diastrophic dysplasia (DTD) is a skeletal dysplasia characterized by short stature, joint contractures, cleft palate, and characteristic clinical signs such as the "hitchhiker" thumb and cystic swelling of external ears. The first mutations in the SLC26A2 gene were found in individuals with DTD.

• Recessive multiple epiphyseal dysplasia (EDM4) is characterized by joint pain (usually the hips and knees), malformations of the hands, feet, and knees, and scoliosis. About 50% of individuals have an abnormal finding at birth, e.g., clubfoot, cleft palate, or cystic ear swelling. Median height in adulthood is in the tenth centile [Superti-Furga, Rossi et al 1996; Superti-Furga et al 1999; Superti-Furga et al 2001].

Natural History

Atelosteogenesis type II (AO2) is usually lethal in the neonatal period because of lung hypoplasia, tracheobronchomalacia, and laryngeal malformations. Pregnancy complications of polyhydramnios may occur.

AO2 is clinically very similar to diastrophic dysplasia (DTD) [Rossi, van der Harten et al 1996].

Newborns with AO2 present with short limbs, adducted feet with wide space between the hallux and the second toe, hitchhiker thumb, cleft palate, and facial dysmorphism. Disproportion between the short skeleton and normal-sized skull is immediately evident; the limb shortening is mainly rhizomelic; the gap between the toes, ulnar deviation of the fingers, and adducted thumbs are typical of sulfate transporter-related dysplasias [Newbury-Ecob 1998, Superti-Furga et al 2001]. The neck is short, the thorax is narrow, and the abdomen protuberant.

Cleft palate is a constant feature, whereas the degree of facial dysmorphism is variable. Midface hypoplasia is usually present, together with a flat nasal bridge and micrognathia. Epicanthal folds, ocular hypertelorism, and low-set ears can also be present.

Spinal scoliosis and dislocation of the elbows are reported [Newbury-Ecob 1998].

Genotype-Phenotype Correlations

Genotype-phenotype correlations indicate that the amount of residual activity of the sulfate transporter modulates the phenotype [Rossi et al 1997] in a spectrum from lethal ACG1B to mild EDM4. Homozygosity or compound heterozygosity for mutations predicting stop codons or structural mutations in transmembrane domains of the sulfate transporter are associated with the more severe phenotype of ACG1B. The combination of a severe mutation (predicting stop codons or structural mutations in transmembrane domains) with a mutation located in extracellular loops, in the cytoplasmic tail of the protein, or in the regulatory 5'-flanking region of the gene results in the less severe phenotypes, i.e., AO2 and DTD [Hastbacka et al 1996; Superti-Furga, Rossi et al 1996; Rossi et al 1997; Karniski 2001; Rossi & Superti-Furga 2001; Karniski 2004].

The most common SLC26A2 mutation outside Finland, p.R279W, is a mild mutation resulting in the EDM4 phenotype when homozygous and mostly in the DTD phenotype when in the compound heterozygous state. In individuals with AO2, the p.R279W mutation was combined with a severe, structural mutation (e.g., p.R178X, delc418 [Rossi, van der Harten et al 1996], or p.N425D [Rossi et al 1997]) whereas in individuals with DTD, p.R279W was combined with a mutation predicting some residual activity (e.g., IVS1+2T>C [Rossi, van der Harten et al 1996] or delV340 [Karniski 2001]). Therefore, the same mutations associated in some individuals with the AO2 phenotype can be found in individuals with DTD if the second allele is a relatively mild mutation, or in individuals with ACG1B if the second mutation is a structural, severe one [Rossi & Superti-Furga 2001].

IVS1+2T>C, the second-most common mutation, is very frequent in Finland ("Finnish" mutation). It produces low levels of correctly spliced mRNA and results in DTD when homozygous.

The mutation p.C653S is the third-most common, with a frequency among DTDST pathogenic alleles very close to that of mutation IVS1+2T>C in non-Finnish populations. It results in EDM4/rMED when homozygous and in EDM4/rMED or DTD when compounded with other mutations.

Mutations 1045-1047delGTT (V340del) and 558C>T (p.R178X) are associated with the severe phenotypes ACG1B and AO2.

Most other mutations are rare, with the possible exception of p.C653S, which has been recognized with increased frequency in individuals with milder DTD and with EDM4.

The same mutations associated in some individuals who have the ACG1B phenotype can be found in individuals with a milder phenotype (AO2 and DTD) if the second allele is a relatively mild mutation. Indeed, missense mutations located outside the transmembrane domain of the sulfate transporter are often associated with a residual activity that can "rescue" the effect of a null allele [Rossi & Superti-Furga 2001].

Nomenclature

The name "atelosteogenesis" was coined by Maroteaux et al (1982) for a different condition.

In 1987, Sillence and colleagues created the term "atelosteogenesis type 2" for a group of fetuses or stillborns who had all previously been diagnosed as having "severe diastrophic dysplasia." The reason was an apparent hypoplasia of the distal humerus and variable fibular hypoplasia (but not aplasia) that was slightly reminiscent of atelosteogenesis type I (AO1). The redefinition of this severe DTD variant as atelosteogenesis type 2 was unfortunate because it suggested a relationship with AO1 and at the same time denied the relationship with diastrophic dysplasia. Later biochemical and molecular studies brought this entity back to its origin, i.e., in the diastrophic dysplasia-achondrogenesis group in which AO2 is considered to be a severe form of DTD, and in which lethality distinguishes AO2 from DTD.

AO2 is currently classified in the "Sulfation disorders group" in the revised Nosology and Classification of Genetic Disorders of Bone [Superti-Furga & Unger 2006].

Prevalence

No data are available on the prevalence of AO2. Among the sulfate transporter-related dysplasias, AO2 is the rarest phenotype.

Evaluations at Initial Diagnosis to Establish the Extent of Disease

• Complete skeletal survey

• Respiratory status

Treatment of Manifestations

• Palliative care for the viable newborn

Testing of Relatives at Risk

Genetic testing in families with an index case is the basis for offering early prenatal testing in future pregnancies.

See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.

Therapies Under Investigation

Search ClinicalTrials.gov for access to information on clinical studies for a wide range of diseases and conditions.

Mode of Inheritance

Atelosteogenesis type II (AO2) is inherited in an autosomal recessive manner.

Risk to Family Members

Parents of a proband

• The parents of an affected child are obligate heterozygotes and thus carry a single copy of a disease-causing mutation in the SLC26A2 gene.

• Heterozygous carriers are asymptomatic and have normal stature.

• No evidence suggests that carriers are at increased risk of developing degenerative joint disease.

Sibs of a proband

• At conception, each sib of a proband with AO2 has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier.

• Once an at-risk sib is known to be unaffected, the chance of his/her being a carrier is 2/3.

• To date, de novo mutations in a proband and germline mosaicism in the parents have not been reported.

Offspring of a proband. AO2 is a perinatally lethal condition; affected individuals do not reproduce.

Other family members. Each sib of the proband's parents is at a 50% risk of being a carrier.

Carrier Detection

• Carrier testing for at-risk family members is available on a clinical basis once the mutations have been identified in the proband.

• Carrier detection in reproductive partners of a heterozygous individual is available on a clinical basis. The partners can be screened for the five most common pathogenic alleles, p.R279W, IVS1+2T>C, p.C653S, delV340, and p.R178X. The risk of carrying an SLC26A2 mutation is reduced from the general population risk of 1:100 to about 1:300 when these five alleles are excluded.

Related Genetic Counseling Issues

Family planning. The optimal time for determination of genetic risk, clarification of carrier status, and discussion of availability of prenatal testing is before pregnancy.

DNA banking. DNA banking is the storage of DNA (typically extracted from white blood cells) for possible future use. Because it is likely that testing methodology and our understanding of genes, mutations, and diseases will improve in the future, consideration should be given to banking DNA of affected individuals. DNA banking is particularly relevant in situations in which the sensitivity of currently available testing is less than 100%. See See DNA Banking for a list of laboratories offering this service.

Prenatal Testing

High-risk pregnancies

• Molecular genetic testing. Prenatal diagnosis for pregnancies at 25% risk is possible by analysis of DNA extracted from fetal cells obtained by chorionic villus sampling (CVS) at about ten to 12 weeks' gestation or by amniocentesis usually performed at about 15-18 weeks' gestation. Both disease-causing alleles of an affected family member must be identified before prenatal testing can be performed.

• Ultrasound examination. Transvaginal ultrasound examination early in pregnancy is a reasonable alternative to molecular prenatal diagnosis because the testing is not invasive. However, the diagnosis can be made with confidence only at week 14-15, and reliability is highly operator dependent.

• Biochemical testing. There are no data on prenatal functional biochemical testing (sulfate incorporation test on chorionic villus or fibroblasts).

Note: Gestational age is expressed as menstrual weeks calculated either from the first day of the last normal menstrual period or by ultrasound measurements.

Low-risk pregnancies

• Routine ultrasound examination. Routine prenatal ultrasound examination may identify very short fetal limbs ± polyhydramnios ± small thorax, raising the possibility of AO2 in a fetus not known to be at risk. Subtle findings on ultrasound examination may be recognizable in the first trimester, but in low-risk pregnancies, the diagnosis of skeletal dysplasia is usually not made until the second trimester.

• Molecular genetic testing. DNA extracted from cells obtained by amniocentesis can theoretically be analyzed to try to make a molecular diagnosis prenatally. However, the differential diagnosis in such a setting is very broad (see Differential Diagnosis).

Preimplantation genetic diagnosis (PGD) may be available for families in which the disease-causing mutations have been identified in an affected family member. For laboratories offering PGD, see

Molecular Genetic Pathogenesis

Mutations in the SLC26A2 (DTDST) gene [Dawson & Markovich 2005] are responsible for the family of chondrodysplasias including achondrogenesis 1B (ACG1B), diastrophic dysplasia (DTD), atelosteogenesis type 2 (AO2), and recessive multiple epiphyseal dysplasia (EDM4) [Hastbacka et al 1996; Superti-Furga, Hastbacka, Rossi et al 1996; Rossi et al 1997; Superti-Furga et al 1999; Superti-Furga 2001; Superti-Furga et al 2001]. Impaired activity of the sulfate transporter in chondrocytes and fibroblasts results in the synthesis of proteoglycans that are not sulfated or are insufficiently sulfated [Satoh et al 1998, Rossi et al 1998], most probably because of intracellular sulfate depletion [Rossi, Bonaventure et al 1996]. Undersulfation of proteoglycans affects the composition of the extracellular matrix and leads to impairment of proteoglycan deposition, which is necessary for proper enchondral bone formation [Corsi et al 2001 , Forlino et al 2005]. A correlation exists between the mutation, the predicted residual activity of the sulfate transporter, and the predicted severity of the phenotype [Rossi et al 1997, Cai et al 1998, Rossi & Superti-Furga 2001, Karniski 2004, Maeda et al 2006].

Normal allelic variants: The coding sequence of the SLC26A2 gene is organized in two exons separated by an intron of approximately 1.8 kb, and encodes a protein of 739 amino acids that is predicted to have 12 transmembrane domains and a carboxy-terminal, cytoplasmic, moderately hydrophobic domain [Hastbacka et al 1994]. A further untranslated exon is located 5' relative to the two coding exons; it has probable regulatory functions, as the mutation IVS1+2T>C (the "Finnish" allele) located in this region was shown to lead to reduced mRNA transcription [Hastbacka et al 1999].

The p.T689S allele has been frequently observed at the heterozygous or homozygous state in several controls of different ethnicities and is thus a common polymorphism [Cai et al 1998, Rossi & Superti-Furga 2001].

Evidence suggests that p.R492W is a rare polymorphism, found in seven out of 200 Finnish controls and in five out of 150 non-Finnish controls [authors, unpublished data]. This allele was erroneously considered pathogenic in previous reports [Rossi & Superti-Furga 2001].

Pathologic allelic variants: Five pathogenic alleles of the SLC26A2 gene appear to be recurrent: p.R279W, IVS1+2T>C, p.C653S, delV340, and p.R178X. Together they represent approximately two-thirds of the pathogenic mutations in SLC26A2. Of the five, the p.R279W, IVS1+2T>C, and p.R178X mutations are associated with the AO2 phenotype [Superti-Furga, Rossi et al 1996; Rossi & Superti-Furga 2001]. In compound heterozygotes, the phenotype associated with each pathogenic allele depends on the combination with the second mutation.

Distinct phenotypes known to be allelic to AO2 are ACG1B, DTD, and EDM4.

Normal gene product: The diastrophic dysplasia sulfate transporter gene SLC26A2 encodes a transmembrane protein that transports sulfate into chondrocytes to maintain adequate sulfation of proteoglycans. The sulfate transporter protein belongs to the family of sulfate permeases. The overall structure with 12 membrane-spanning domains is shared with two other human anion exchangers: PDS, a chloride-iodide transporter involved in Pendred syndrome and CLD, which is responsible for congenital chloride diarrhea. The function of the carboxy-terminal hydrophobic domain of SLC26A2 is not yet known. The SLC26A2 gene is expressed in developing cartilage in human fetuses but also in a wide variety of other tissues [Haila et al 2000, Haila et al 2001]. The size of the predominant mRNA species is greater than 8 kb, indicating the existence of significant untranslated sequences [Hastbacka et al 1994, Hastbacka et al 1999].

Abnormal gene product: Most of the SLC26A2 mutations either predict a truncated polypeptide chain or affect amino acids that are located in transmembrane domains or are conserved in man, mouse, and rat. Individuals homozygous for the "Finnish" mutation IVS1+2>C have reduced levels of mRNA with intact coding sequence [Rossi, van der Harten et al 1996]. Thus, the mutation presumably interferes with splicing and/or further mRNA processing and transport [Hastbacka et al 1994, Hastbacka et al 1999].

The p.R178X and delV340 mutations were shown to abolish sulfate transporter activity in a Xenopus oocyte model [Karniski 2001] and in a HEK-293 cell culture model [Karniski 2004], respectively.

Literature Cited

Published Statements and Policies Regarding Genetic Testing

No specific guidelines regarding genetic testing for this disorder have been developed.

Revision History

• 28 December 2006 (me) Comprehensive update posted to live Web site

• 21 July 2004 (me) Comprehensive update posted to live Web site

• 30 August 2002 (me) Review posted to live Web site

• 1 March 2002 (lb) Original submission